The present invention relates to a method of and an apparatus for manufacturing float glass.
There has been known a method of manufacturing a glass plate having a thickness smaller that the equilibrium thickness of the glass plate by using a float process. Namely, molten glass is supplied to a molten metal bath to form a glass ribbon, and the glass ribbon is subjected to a pulling force on the molten metal bath to be stretched. Since the glass ribbon tends to contract in its width direction when being stretched, a pair of top rollers are provided so that both edges of the glass ribbon are in contact with the top rollers to prevent the glass ribbon from contracting. The elogation of the glass ribbon is carried out at a relatively low temperature such as 950.degree. C.-800.degree. C. to make the use of the top rolls effective. However, such low temperature increases the viscosity of glass (in Na.sub.2 O-CaO-SiO.sub.2 series glass used for window glasses for buildings, log .eta. (the viscosity of glass in the unit of poise) is about 4.8-6.5). Therefore, there was a problem that a long distance was required for the float glass to reduce its thickness from the equilibrium thickness to a target thickness (for instance, when the target thickness is 1 mm, the length is about 20 m). As a result, the size of a vessel containing the molten glass bath became large and a large amount of energy was lost. Further, in this case, job efficiency for glass products decreases since both edges of the glass ribbon are very thick.
Japanese Examined Patent Publication No. 6844/1975 discloses a method of forming a thin glass ribbon wherein a standing wave is formed on the molten metal bath and the glass ribbon is moved beyond a projection formed by the standing wave. However, a slight change in the standing wave largely affects the thickness of the glass ribbon. Accordingly, it is difficult to control the thickness of the glass ribbon. Further, since the standing wave is formed by raising a part of the surface of a molten metal, it is difficult to form the standing wave having a stable shape. Accordingly, distortion results in the glass ribbon, whereby uniform thickness of the glass ribbon can not be obtained. In the method of using the standing wave, since the glass ribbon has a thickness thinner than the target thickness when the glass ribbon is passed beyond the reach of the standing wave, the glass ribbon is easily broken. As a method of forming the float glass having a thickness thicker than the equilibrium thickness, a ribbon width control means such as a carbon fender or the top rollers is provided at both sides of the vessel so that the shape of the glass ribbon between a lip and the ribbon width control means is detected, and the content of glass falling from the lip is adjusted. However, there arise such problems that efficiency for working is low.